‘Low temperature’ processing with electric field assisted sintering

Shrinkage during electric field assisted sintering can lead to debonding and cracking, as shown this sample of 8YSZ sintered at 840°C. Credit: Kim; JACerS; Wiley.

When I started writing this, I was on my way—literally, at 34,000 feet somewhere over Arizona—to the PACRIM-GOMD meeting this week in San Diego. While there I will be dropping in on a number of symposia, and one that I am looking forward to in particular is the symposium in honor of Zuhair Munir. Munir recently assumed professor emeritus status in the Department of Chemical Engineering and Materials Science at the University of California, Davis.

Munir is widely recognized for his research on sintering, in particular approaches to densification that take advantage of enhancements, such electric field assisted sintering and spark plasma sintering. Therefore, it is not surprising that many of the talks in the symposium are sintering related.

What is surprising is the coincident publication of several papers in the May issue of the Journal of the American Ceramic Society on electric field assisted sintering, two of which look at the same material, 8 mol% yttria stabilized zirconia, or 8YSZ.

One of those papers is part of a series of reports on a study investigating the mechanisms of electric field assisted sintering. The work was done by Kim, et al. at the University of Pennsylvania and the Korea Advanced Institute of Science and Technology (Daejeon, Korea). The long-term goal of the study is to understand how electric field affects microstructure development and stability, with a particular interest in the mechanisms of how the electric field interacts with point and planar defects.

While electric field assisted sintering is interesting in its own right as a densification process, it merits exploration more generally because of its relevance to materials where the service environment is both high temperature and under load. The authors give examples including heating elements, solid oxide fuel cells, and solid oxide electrolysis cells.

The researchers studied 8YSZ because its high oxygen ion conductivity helps prevent resistance heating, especially at disruptive microstructural features such as necks, pores, and grain boundaries. Also, cation diffusivity in 8YSZ is low, which stabilizes the microstructure in high temperature environments. They say in the paper, “In all, 8YSZ is a rather ideal model ceramic for studying electric loading effects on sintering, grain growth and other related aspects of microstructure evolution.”

Samples ranged in thickness from 0.27–1.40 mm and were wound with platinum electrodes. At 1,100˚C, “thick” samples sintered without an applied field did not densify. Increasing the temperature to 1,300˚C did not help much—the samples were less than 95 percent dense after 1,000 minutes. By applying a field of 10 A/cm2 during sintering at 1,100˚C, samples were 95 percent dense after 1,000 minutes. “Similar” densification resulted from applying a field of 20 A/cm2 at 1250˚C.

The paper includes an interesting graph of densification as a function of time. In the absence of applied field, densification plateaus in less than 500 minutes of sintering, regardless of temperature. Higher sintering temperatures yield higher relative densities, but there is a limit beyond which more time at temperature is futile. However, when sintering at 1,100˚C under a field of 10 A/cm2, there is a rapid densification regime followed by a less rapid densification regime, but relative densification does not plateau. They also showed that densification under applied electric field is achievable under “unprecedented low temperature, such as 850˚C.”

The authors suggest a two-part mechanism for densification: “electro-migration of pores” to the surface, and surface diffusion of cations (which is important for pore migration, too). At higher temperatures, they suggest that pores migrate to the grain boundaries, where they can diffuse more quickly.

There is much more to it, of course. Major themes this week revolve around materials problems related to sustainability, energy issues, and environment. The potential of field assisted sintering raises interesting questions about low temperature processing of highly refractory materials. I’m hoping to learn more about it this week.